(1) Field of the Invention
The present invention relates to fault protection circuitry for providing protection to a power switching device and more particularly to a high speed means for protecting the power switching device against faults caused by short circuits, oversaturation, shoot through, and undervoltage.
(2) Description of the Prior Art
Alternating current electric motor systems often depend on batteries as a power source, and, thus, the design of high power, power dense, reliable inverter electronics is critical in converting direct current from batteries to alternating current for supply to the electric motor. The development of high power, metal oxide semiconductor (MOS) controlled switching devices such as an insulated gate bipolar transistor (IGBT) has enabled the use of semiconductor devices in power control circuitry. However, MOS controlled switching devices are sensitive to varying voltages and must be protected from voltage spikes and voltage underflow. Excess current through the MOS switching device causes high temperatures in the device which eventually will lead to its destruction. Operation of the MOS switching device at low gate drive voltage causes the gate to drop out of saturation. When the device is out of saturation, heating occurs in the device because of increased resistivity resulting in damage to the device.
As a general rule, once any of the above failure modes appears, the MOS controlled switching device should be turned off within 10 .mu.sec. If this time limit is exceeded the switching device can be destroyed or severely weakened. Furthermore, the MOS switching device must be shut down properly to avoid placing the device in a failure mode. For example, trying to shut down the device while it is switching full power can cause destructive current and voltage spikes.
Various prior art circuits exist for detecting voltage spikes and underflow and providing an alternate path for excess power. To detect voltage spikes and underflows, it is well known to provide a reference voltage which can be compared with the voltage across the collector and emitter of the MOS device to activate a switch shutting down power to the MOS device.
Other prior art devices such as U.S. Pat. No. 4,893,212 provide an alternate path by connecting the collector of a bipolar transistor to the lower voltage switch terminal of a MOS device, the base of the transistor is connected to the higher voltage switch terminal of the MOS device, and the emitter of the transistor is connected to a reference voltage. The bipolar transistor allows current to flow around the MOS device when an excess voltage exists on the base. The '212 patent also includes the use of a Zener diode and resistor in parallel with the gate terminal and lower switch terminal of the MOS device. When the gate terminal is subjected to a preset excess voltage, the Zener diode allows flow of reverse current through the diode to prevent gate oxide breakdown of the MOS device.
Prior art gate control circuits have the capability of sensing that a switching device is entering a failure mode, but these gate drive circuits are not quick enough to shut down the gate before the switching device is destroyed. Furthermore, many prior art fault protection circuits are placed in the flow of switched current making them susceptible to the same failure modes as the switching device, and thus, the same voltage surge that results in destruction of the MOS device will also destroy the fault protection device. Previous gate drive designs also shut down a switching device suddenly without providing protection against transients inherent in device shut down.